Lately I’ve been fantasizing about my dream house, as well as musing on unconventional building designs. Perhaps I have hobbit blood; for me, the idea of a cozy and inviting dwelling is one without many right angles, giving it an organic, somewhat cavelike feel. Although I have always lived in buildings with conventional vertical walls, I picture a home in which each room is a different shape, with rounded corners, curved ceilings, and angled doors. In a concession to gravity, I’ll leave the floor flat and level for the most part. Without a doubt, at least one room must be completely circular—no matter how hard it is to buy furniture or hang artwork. And of course, my ideal home would be sturdy, secure, economical to build, and highly energy-efficient.
All these tastes would seem to make me an excellent candidate to own a monolithic concrete dome house. Like geodesic domes, this type of building encloses a large amount of area with a minimum of material and offers a distinctive, unconventional shape. Despite these similarities, the two types of dome building are very, very different when it comes to construction materials and techniques, not to mention some of the characteristics of the finished product.
A Dome Idea
The word “monolithic” in this sense simply means “in one piece”—not necessarily massive. As compared to geodesic domes constructed out of hundreds of triangular pieces of wood or metal, a monolithic concrete dome is a single, contiguous surface of reinforced concrete. What makes these domes particularly noteworthy is the modern method of building them.
In decades past, building a concrete dome meant creating a pile of dirt in the desired size and shape, pouring concrete over it, and then digging out the inside after the concrete had set. This was a long and labor-intensive process—and it required you to have a sufficient quantity of earth at your disposal as well as the means of moving (and removing) it. Thanks to a patented process invented in the late 1970s, monolithic concrete domes can be built in a matter of days without any heavy equipment at all. The secret is to build it from the outside in.
The process begins with a fairly ordinary concrete foundation—typically in the shape of a ring. A heavy-duty, dome-shaped “balloon” known as an airform is affixed to the foundation and inflated by special fans. Using a spray pump, the builder applies a thick layer of polyurethane foam to the inside of this balloon; the foam provides insulation as well as a bit of structural support for the remainder of the building process. Next, the builder attaches a steel rebar framework to the inside of the foam, and finally applies a sprayable concrete mixture known as shotcrete. The shotcrete reaches a thickness of about 3 inches (8cm), embedding the rebar and forming a reinforced concrete shell. Then the fan is turned off and the interior and exterior of the dome are finished using conventional materials (such as stucco). The airform, by the way, remains permanently in place, serving as an extra moisture barrier on the outside of the dome.
Concrete domes are strong—highly resistant to damage by earthquake, hurricane, or wind (even tornadoes). Because concrete is not flammable, the shell itself is fireproof, and also invulnerable to termites and other pests. And since concrete is a good insulator, monolithic concrete domes are extremely energy-efficient. Although the materials used to build a concrete dome are expensive, you need relatively little of them, so the cost of a monolithic concrete dome is comparable to that of a wood frame building of similar size. Since the cost savings increase with the size of the building, concrete domes are becoming an increasingly popular choice for churches, gymnasiums, arenas, storage facilities, and even airplane hangars.
For all their benefits, though, concrete domes are by no means perfect. For instance, they tend to trap moisture inside, making a dehumidifier or air conditioner mandatory except in very dry climates. And of course they have the same problems all dome houses have. Their geometry does not work well in narrow urban lots. Furnishing, decorating, and cleaning a dome home can be challenging. You may have difficulty obtaining financing or insurance for such a nonstandard design. And you may have to contend with neighbors who are concerned about their own homes’ resale value with that alien spacecraft parked next door.
But a “dome” need not look that unusual after all. The airforms used to create monolithic concrete domes can be made in nearly any shape. Although you probably wouldn’t use them to make a cube, there’s no reason a building made in this fashion has to be a perfect dome either. A single custom-made airform can also be used to make a series of interconnected domes, and can include extensions, insets, cutouts, and augmentations—making it possible to integrate doorways, window frames, carports, or anything else you’d like into the shell of your building. Domes can even have multiple levels.
On that hypothetical future day when I can afford to build the home of my dreams, I may or may not choose a monolithic concrete dome. Pyramids have a lot going for them too, and then there’s always the classic elegance of a castle. I’ll cross that drawbridge when I get to it.
Note: This is an updated version of an article that originally appeared on Interesting Thing of the Day on August 3, 2004.